Vacuum gauge readings

Discussion in 'Troubleshooting & Diagnosis' started by Chuck71RS, Apr 22, 2021.

  1. Chuck71RS

    Chuck71RS Veteran Member Lifetime Gold Member

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    Initial timing 14 BTDC, Mallory HEI distributor, fresh AC R45TS plugs (.045" gap), added fresh gas (91 octane), ported distributor vacuum advance, VA set to Mallory spec for starting point. 600cfm Edelbrock, Edelbrock Performer intake

    Comp Cam 12-238-2 has 46 degrees of valve overlap.
    https://www.compcams.com/xtreme-energy-218-224-hydraulic-flat-cam-for-chevrolet-small-block.html

    Engine starts easy, cold or warm

    At fully warmed had the below vacuum readings (Autometer Z-series vacuum gauge)
    850 (idle) 12" constant fluctuation but slow up to 1". Every so often a slight-brief drop
    1500: 15.5" +/- 0.5" (mechanical advance kicked in at 1300 with 5 degrees
    2000: 19" steady
    2500: 20" steady.
    Vacuum never increased above 20". When throttle fully released, vacuum jumped 2" briefly then settled down to idle

    What does the vacuum readings indicate? Is the cam choppy idle causing low and fluctuating vacuum at idle?
     
    Last edited: Apr 22, 2021
  2. cadillac_al

    cadillac_al Veteran Member

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    The cam doesn't sound that radical to me. I would give the distributor full vacuum to see if it runs better anyway. I only use ported vacuum as a crutch for some stubborn high idling engines.
     
  3. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Vacuum readings don't seem far off to me for a carburetor engine with a cam on 110 C/L, even with that low I/E duration. Vacuum fluctuation at idle is due to overlap on your cam. If you want more vacuum at idle, first use the vacuum gauge and adjust idle screws to highest idle vacuum. You can also increase idle timing but that will bump up total timing. This is where you will need find a way to limit mechanical advance in that Mallory distributor. I've tried/used a variety of objects as limiters, some worked, some didn't. You can buy a MSD distributor that you can fully control mechanical advance. If it's in your budget, the best thing you can buy is an Edlebrock ProFlo 4, then you can adjust initial advance from your Android device, via Bluetooth. I'm currently set at 20*, have much larger cam, and get about 15", engine hot, in neutral, 875 rpm. Good luck.
     
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  4. Chuck71RS

    Chuck71RS Veteran Member Lifetime Gold Member

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    I don't know enough to know if I need more idle vacuum. Any thoughts what the results would be from increased idle vacuum? What if I move the vacuum advance line from ported to manifold vacuum?
     
    Last edited: Apr 23, 2021
  5. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Results of increased vacuum is a smoother idle and better throttle response off idle. With that cam you should be able to use full time, not ported, vacuum advance. Disconnect and plug vacuum advance, adjust timing to 14*, then adjust idle screws on carburetor to highest vacuum reading. Connect vacuum advance to full time vacuum and your idle should increase. Adjust idle back down to 850 and see how it performs.
     
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  6. COPO

    COPO Veteran Member Lifetime Gold Member

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    You want the highest vacuum reading when adjusting the mixture screws.
     
  7. biker

    biker Veteran Member

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    Exactly.
     
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  8. Chuck71RS

    Chuck71RS Veteran Member Lifetime Gold Member

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    Switched from ported to full vacuum. Also removed the new plugs and cleaned them. New plugs only started twice before working on the timing yet they were black. Started the engine and the vacuum had increased 2"

    Backed the car on to the short street in front of the house and the engine ran pretty good. Still needs a bit more but good for the safety inspection
     
  9. biker

    biker Veteran Member

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    Nice. Perfection comes from putting the time in on the small stuff now. Fine tune timing, vac advance and idle mixtures/speed.
     

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